1. Field of the Invention
The present invention relates to an apparatus and method for optimizing the size of a memory in a Mobile Station (MS). More specifically, the present invention relates to an apparatus and method for optimizing the size of a buffer that an MS allocates for soft-combining of retransmitted data when it operates in a High-Speed Shared Control Channel-less mode (HS-SCCH-less mode) according to an asynchronous mobile communication system standard, the 3rd Generation Partnership Project (3GPP) REL-7 standard.
2. Description of the Related Art
The HS-SCCH-less mode is defined in the 3GPP REL-7 standard. The HS-SCCH-less mode is a technique in which, when small-size Transport Blocks (TBs) are transmitted, their initial transmission version is transmitted without the HS-SCCH, to thereby increase cell capacity.
FIG. 1 illustrates a conventional operation for transmitting channels in the HS-SCCH-less mode.
Referring to FIG. 1, a Base Station (BS) transmits initial transmission data to an MS without using an HS-SCCH, i.e. in the HS-SCCH-less mode. That is, upon receipt of Channel Quality Information (CQI) from the MS in step A), the BS transmits initial High-Speed Downlink Shared CHannel (HS-DSCH) data to the MS on a High-Speed Physical Downlink Shared CHannel (HS-PDSCH) without using the HS-SCCH in step B).
The HS-PDSCH transmission has the following features in the HS-SCCH-less mode. Only Quadrature Phase Shift Keying (QPSK) is used as a modulation scheme and only four TB formats or sizes are available. The BS notifies the MS of these TB sizes before transmitting the HS-PDSCH. The HS-PDSCH has a 24-bit Cyclic Redundancy Check (CRC) that is MS-specific like that of the HS-SCCH.
Up to two HS-PDSCH channelization codes are available to the MS. The BS preliminarily allocates an HS-PDSCH channelization code before transmitting the HS-PDSCH to the MS and the MS receives the HS-PDSCH using the allocated HS-PDSCH channelization code. Then the MS performs blind decoding on the HS-PDSCH by sequentially applying the four TB sizes to the HS-PDSCH. If the CRC of the HS-PDSCH indicates an error during the decoding, the MS requests a retransmission of the HS-PDSCH. However, there are restrictions regarding Hybrid Automatic Repeat reQuest (HARQ) as a retransmission scheme as follows.
Up to two retransmissions are allowed. Redundancy Versions (RVs) that are available are semi-fixedly defined according to retransmission numbers. Table 1 below lists transmission numbers versus redundancy versions.
TABLE 1Transmission numberRV (Redundancy Version)Initial transmission0Second transmission (first3retransmission)Third transmission (second4retransmission)
When a retransmission is requested in the HS-SCCH-less mode, the BS transmits the following information to the MS on an HS-SCCH of type 2.
(1) 2-bit TB size Information indicating the size of a retransmitted HS-DSCH TB;
(2) a 3-bit Pointer to the previous transmission (PTR) indicating the position of the previously transmitted data, i.e. the position of an Incremental Redundancy (IR) buffer at which there is data to be soft-combined with a retransmitted bit stream; and
(3) 1-bit second or third transmission information indicating a current retransmission number. If the second or third transmission information is 0, this indicates a second transmission and if the second or third transmission information is 1, this indicates a third transmission.
FIG. 2 is a timing diagram of a conventional MS operation.
Referring to FIG. 2, after entering the HS-SCCH-less mode, the MS operates as follows.
First, the MS demodulates a received HS-PDSCH and determines whether the HS-PDSCH is type 1. If the HS-PDSCH is neither type 1 nor type 2, the MS blind-decodes the HS-PDSCH with respect to all available TB sizes. Herein, type 2 means retransmission.
If the CRC of the blind-decoded HS-PDSCH is good, the MS provides packets included in the HS-PDSCH to a Medium Access Control (MAC) layer.
On the other hand, if the blind-decoded HS-PDSCH has a bad CRC with respect to every TB size, the MS buffers the current data, i.e., second rate-dematched data with respect to every TB size at a position computed by equation (1), for soft-combing with later retransmitted data in step A).IR buffer offset=[5×CFN+subframe number] mod 13  (1)where CFN represents Connected Frame Number (CFN).
Then the MS demodulates a received HS-SCCH and, if the HS-SCCH is type 2, determines that a retransmission takes place in the HS-SCCH-less mode.
The MS calculates parameters for rate dematching and channel coding of the currently received data by detecting a TB size and a retransmission number from information included in the HS-SCCH in step B).
The MS calculates the IR buffer offset of the initial transmission using a PTR acquired from the HS-SCCH of type 2, a current CFN, and a current subframe number byPrevious IR buffer offset=[5×CFN+subframe number−6−PTR] mod 13  (2)where PTR represents “Pointer to the previous transmission”.
After second-rate dematching of the current received data, the MS calculates the previous IR buffer offset by equation (2) and soft-combines the second rate-dematched data of the current bit stream with the data buffered at a position of the IR buffer determined by the TB size in step C).
The MS performs first-rate dematching and channel decoding on the soft-combined data and checks the CRC of the channel-decoded data.
If the CRC indicates no error, the MS provides the decoded data to the MAC layer. Otherwise, the MS requests and waits for a retransmission.
In summary, the MS subjects initial transmission data to blind decoding and sequentially stores second rate-dematched data of the initial transmission data in the IR buffer according to an IR buffer offset and all available TB sizes.
If the initial transmission data turns out bad based on a CRC check with respect to every TB size, the MS keeps the buffered data in the IR buffer. Then, the MS demodulates a received HS-SCCH and if the HS-SCCH is type 2, it computes an IR buffer offset and offsets of TBs of all TB sizes, for soft combining.
The MS performs second-rate dematching on currently received data and soft-combines the second rate-dematched data with the data at the buffer position. Then the MS performs first-rate dematching and channel decoding on the combined data.
For satisfactory operation, the IR buffer should meet certain requirements based on the following parameters. The MS applies blind decoding to initial transmission data with respect to every TB size and stores all of second rate-dematched data of the blind-decoded data in the IR buffer, for soft combining with retransmitted data.
Hence, sufficient memory areas should be allocated to buffer second rate-dematched data for all (four) TB sizes during one Transmission Time Interval (TTI). That is, the IR buffer should be able to store the data resulting from blind decoding of 13 successive TTIs for all TB sizes.
If a CRC check indicates that the currently received data from the BS is bad despite blind decoding for every TB size, the MS stores the currently received data for soft-combining with a retransmitted TB.
Since a maximum TB size is 1483 bits, a buffer size that the MS requires in the HS-SCCH-less mode is 231348 bits (=1483 bits×3 (coding rate)×13 (the number of monitoring TTIs)×4 (the number of available TB sizes)).
The 231348-bit buffer size imposes a constraint on the MS when receiving data. Accordingly, there exists a need for an apparatus and method for reducing the buffer size requirement.